This invention relates to a ground subsiding wellhead. More particularly, the invention relates to a wellhead for use in a system for reestablishing tension in the casing on which the wellhead is mounted when tension has been lost due to ground subsidence.
A wellhead supports a production string. The string includes a production casing, through which petroleum reacnes the surface. The casing is in tension to avoid its buckling under its own weight. Tension is maintained by a lower wellhead assembly at the surface of the ground. The bottom of the casing is generally fixed to the bottom of the well by cementing or the like. The extraction of petroleum from the well reduces ground support, and the ground subsides. The lower wellhead subsides too, reducing the tensile stress in the production string, even reversing it to compression. This is described in an article in the Journal of Petroleum Technology ("A Solution to Ground Subsidence Problems in Casing Strings and Wellheads," Burley, J.D. et al., pp. 654-660, June 1971).
One solution to the subsidence problem is to excavate below the wellhead and raise the entire wellhead by hydraulic jacks to retension the casing. This solution is undesirable in that it requires excavating a hole and disconnecting cellar and flow lines, among other reasons.
To improve upon this method, the article suggests a wellhead of a separate lower casing head and an upper tubing head, and retensioning the casing as needed by hydraulically jacking the upper tubing head with respect to the lower casing head. In particular, a standard tubing head is modified by threading it onto the top of the casing and leaving a space between the tubing head and a packoff flange of the casing head. The casing head mounts the pack off flange above a hanger packoff. A casing hanger has a slip of segments that have conical outer surfaces and teeth on their inside to engage the casing. The conical outer surfaces of the slip segments engage a complementary conical surface of a bowl of the casing hanger. The casing hanger has a plate bolted to the top of the bowl. Springs between this plate and the slip segments force the slip segments downward into engagement with the casing. The entire casing hanger is supported by the casing head but lifts off the support to close a space between the hanger and structure of the casing head that does not move, specifically a seal assembly, when retensioning the casing.
U.S. Pat. Nos. 3,738,426 and 3,741,297 disclose a similar wellhead and retensioning method.
The described wellheads lift the casing hanger off the casing head during retensioning. When the hydraulic jacks release the casing hanger, it moves down to seat on the casing head. This movement of the casing hanger relieves some of the tension, making retensioning inexact.